powerpc/86xx: Add device_type entry to soc for ppc9a

The 'device_type = "soc";' line *is* needed in the DTS for get_immrbase()
to return the correct address.

Signed-off-by: Martyn Welch <martyn.welch@gefanuc.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>